May your reign last for a thousand, nay, eight thousand generations, until pebbles grow into boulders and moss covers them.
eight thousand years
Original literal meaning; now mainly encountered in classical contexts or as an etymological note.
The word 八千代 literally means eight thousand years.
永遠 is the standard modern word for eternity, while 八千代 is a poetic, classical term used mainly in wishes for long life or traditional verse.
千代 also means 'a thousand years/eternity' and appears far more often in poetry and personal names than the rarer 八千代.
for millennia and millennia; forever and ever
From 八 (eight) + 千 (thousand) + 代 (generation/age), literally 'eight thousand generations', used hyperbolically to mean an immeasurably long time.